Interference mitigation in heterogeneous wireless communication networks

ABSTRACT

A method in a wireless terminal transceiver includes receiving a sequence of frames from a first base station, wherein each frame in the sequence contains a first set of time-frequency resources which may be used for scheduling data and a second set of time-frequency resources not used for scheduling data. The transceiver also receives a message from the first base station identifying a third set of time-frequency resources that is a subset of the first set of time-frequency resources, and estimates the channel state based on the transmission received in the third set of time-frequency resources.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a non-provisional application of U.S.provisional Application No. 61/258,968 filed on 6 Nov. 2009, thecontents of which are incorporated by reference herein and from whichbenefits are claimed under 35 U.S.C. 119.

FIELD OF THE DISCLOSURE

The present disclosure relates to wireless communications and, morespecifically, to spectral efficiency optimization via interferencecontrol and mitigation in heterogeneous networks comprising macro-cellsand home-base stations or femto-cells.

BACKGROUND

Some wireless communication networks are completely proprietary, whileothers are subject to one or more standards to allow various vendors tomanufacture equipment for a common system. One standards-based networkis the Universal Mobile Telecommunications System (UMTS), which isstandardized by the Third Generation Partnership Project (3GPP). 3GPP isa collaborative effort among groups of telecommunications associationsto make a globally applicable third generation (3G) mobile phone systemspecification within the scope of the International MobileTelecommunications-2000 project of the International TelecommunicationUnion (ITU). The UMTS standard is evolving and is typically referred toas UMTS Long Term Evolution (LTE) or Evolved UMTS Terrestrial RadioAccess (E-UTRA).

According to Release 8 of the E-UTRA or LTE standard or specification,downlink communications from a base station (referred to as an “enhancedNode-B” or simply “eNB”) to a wireless communication device (referred toas “user equipment” or “UE”) utilize orthogonal frequency divisionmultiplexing (OFDM). In OFDM, orthogonal subcarriers are modulated witha digital stream, which may include data, control information, or otherinformation, so as to form a set of OFDM symbols. The subcarriers may becontiguous or non-contiguous and the downlink data modulation may beperformed using quadrature phase shift-keying (QPSK), 16-ary quadratureamplitude modulation (16QAM), or 64QAM. The OFDM symbols are configuredinto a downlink sub frame for transmission from the base station. EachOFDM symbol has a temporal duration and is associated with a cyclicprefix (CP). A cyclic prefix is essentially a guard period betweensuccessive OFDM symbols in a sub frame. According to the E-UTRAspecification, a normal cyclic prefix is about five (5) microseconds andan extended cyclic prefix is about 16.67 microseconds. The data from theserving base station is transmitted on physical downlink shared channel(PDSCH) and the control information is signaled on physical downlinkcontrol channel (PDCCH).

In contrast to the downlink, uplink communications from the UE to theeNB utilize single-carrier frequency division multiple access (SC-FDMA)according to the E-UTRA standard. In SC-FDMA, block transmission of QAMdata symbols is performed by first discrete Fourier transform(DFT)-spreading (or precoding) followed by subcarrier mapping to aconventional OFDM modulator. The use of DFT precoding allows a moderatecubic metric/peak-to-average power ratio (PAPR) leading to reduced cost,size and power consumption of the UE power amplifier. In accordance withSC-FDMA, each subcarrier used for uplink transmission includesinformation for all the transmitted modulated signals, with the inputdata stream being spread over them. The data transmission in the uplinkis controlled by the eNB, involving transmission of scheduling grants(and scheduling information) sent via downlink control channels.Scheduling grants for uplink transmissions are provided by the eNB onthe downlink and include, among other things, a resource allocation(e.g., a resource block size per one millisecond (ms) interval) and anidentification of the modulation to be used for the uplinktransmissions. With the addition of higher-order modulation and adaptivemodulation and coding (AMC), large spectral efficiency is possible byscheduling users with favorable channel conditions. The UE transmitsdata on the physical uplink shared channel (PUSCH). The physical controlinformation is transmitted by the UE on the physical uplink controlchannel (PUCCH).

E-UTRA systems also facilitate the use of multiple input and multipleoutput (MIMO) antenna systems on the downlink to increase capacity. Asis known, MIMO antenna systems are employed at the eNB through use ofmultiple transmit antennas and at the UE through use of multiple receiveantennas. A UE may rely on a pilot or reference signal (RS) sent fromthe eNB for channel estimation, subsequent data demodulation, and linkquality measurement for reporting. The link quality measurements forfeedback may include such spatial parameters as rank indicator, or thenumber of data streams sent on the same resources; precoding matrixindex (PMI); and coding parameters, such as a modulation and codingscheme (MCS) or a channel quality indicator (CQI). For example, if a UEdetermines that the link can support a rank greater than one, it mayreport multiple CQI values (e.g., two CQI values when rank=2). Further,the link quality measurements may be reported on a periodic or aperiodicbasis, as instructed by an eNB, in one of the supported feedback modes.The reports may include wideband or subband frequency selectiveinformation of the parameters. The eNB may use the rank information, theCQI, and other parameters, such as uplink quality information, to servethe UE on the uplink and downlink channels.

A home-basestation or femto-cell or pico-eNB or relay node (RN) isreferred to as hetero-eNB (HeNB) or a hetero-cell or hetero base stationin the sequel. A HeNB can either belong to a closed subscriber group(CSG) or can be an open-access cell. HeNBs are used for coverage in asmall area (such as a home or office) in contrast with eNBs (alsoreferred to as macro eNBs or macro-cells) which are typically used forcoverage over a large area. A CSG is set of one or more cells that allowaccess only to a certain group of subscribers. HeNB deployments where atleast a part of the deployed bandwidth (BW) is shared with macro-cellsare considered to be high-risk scenarios from an interferencepoint-of-view. When UEs connected to a macro-cell roam close to a HeNB,the uplink of the HeNB can be severely interfered with particularly whenthe HeNB is far away (for example >400 m) from the macro-cell, thereby,degrading the quality of service of UEs connected to the HeNB. Theproblem is particularly severe if the UE is not allowed to access theHeNB that it roams close to (for example, due to the UE not being amember of the CSG of the HeNB). However, even if the UE roams close to aHeNB that it is allowed to access, the interference can be substantial.Currently, the existing Rel-8 UE measurement framework can be made useof to identify the situation when this interference might occur and thenetwork can handover the UE to an inter-frequency carrier which is notshared between macro-cells and HeNBs to mitigate this problem. However,there might not be any such carriers available in certain networks tohandover the UE to. Further, as the penetration of HeNBs increases,being able to efficiently operate HeNBs on the entire available spectrummight be desirable from a cost perspective. Several other scenarios arelikely too including the case of a UE connected one HeNB experiencinginterference from an adjacent HeNB or a macro cell. The following typesof interference scenarios have been identified.

HeNB (aggressor)→MeNB (victim) downlink (DL)

HUE (aggressor)→MeNB (victim)uplink (UL)

MUE (aggressor)→HeNB (victim)UL

MeNB (aggressor)→HeNB (victim)DL

HeNB (aggressor)→HeNB (victim)on DL

HeNB (aggressor)→HeNB (victim)on UL.

In this disclosure, we discuss HeNB uplink (UL) interference anddownlink (DL) interference problems in further detail and propose amethod that can enable a more effective co-channel/shared channeldeployment of HeNBs in LTE Rel-9 systems and beyond.

The various aspects, features and advantages of the disclosure willbecome more fully apparent to those having ordinary skill in the artupon a careful consideration of the following Detailed Descriptionthereof with the accompanying drawings described below. The drawings mayhave been simplified for clarity and are not necessarily drawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which together with the detailed description below are incorporatedin and form part of the specification, serve to further illustratevarious embodiments and to explain various principles and advantages allin accordance with the embodiments of the present invention.

FIG. 1 is a schematic diagram of a macro-cell and a home-base station inthe macro-cell's coverage area wherein the home-base station downlinktransmission to its UE interferes with UE connected to the macro-cell.

FIG. 2 is a schematic diagram of a macro-cell and a home-base station inthe macro-cell's coverage area wherein a UE transmission to themacro-cell interferes with the home-base station uplink.

FIG. 3 illustrates a diagram showing the bandwidth arrangement in E-UTRAnetwork (E-UTRAN) downlink.

FIG. 4 shows a flow chart of UE receiving a measurement pattern,measuring load/interference on that pattern and either reporting it ortriggering reselection.

DETAILED DESCRIPTION

Methods of a wireless communication device and a wireless base stationare disclosed. The wireless communication device is served by a servingbase station and receives from a neighbor base station a downlinktransmission including a broadcast signal.

In a heterogeneous network comprising macro cells and HeNBs that haveoverlapping bandwidth (BW) deployments, certain interference problemscan arise. One such interference problem is depicted in FIG. 1, wherethe downlink (DL) transmission from a macro-eNB (MeNB) to a UE that isclose to (i.e., within signal range of) a HeNB interferes with DLtransmissions to UEs connected to the HeNB. Another situation is shownin FIG. 2 where the uplink (UL) transmission from a UE connected to amacro-eNB that is close to a HeNB severely interferes with the UL of aUE connected to the HeNB. This case has been identified as interferencescenario 3 in 3GPP TR 25.967 “Home Node B Radio Frequency (RF)Requirements (FDD) (Release 9)” in Universal Terrestrial Radio Access(UTRA) network.

The concept of coordinated multipoint transmission where multipletransmitters schedule UEs such that the transmissions are orthogonal intime-frequency, or there are statistical scheduling gains, are wellknown. The tight co-ordination required for coordinated multipointtransmission requires fast signaling between the multiple transmitters.HeNBs may not have X2 interfaces to other/macro eNBs (for reasons ofcost, complexity and ease of installation) which makes tightco-ordination difficult. Certain ad-hoc techniques where the macro-cellblanks a number of subframes on DL while the load is transferred toHeNBs have been proposed in the 3GPP contribution R1-083195. However,this approach might not be feasible for widespread use as blanking isincompatible with LTE Rel-8 standard, and there might be large coverageholes as initial HeNB deployment will be sparse and the macro-cell willneed to serve the UEs outside the range of HeNBs.

The method is inefficient since the blank sub-frames of the macro cellhave to be adequate to support the needs of all HeNBs within the macrocell's coverage. Also, the necessary co-ordination between a Macro-celland HeNB to enable dynamic allocation of blank subframes necessary tosupport retransmissions for HARQ processes in the CSG does not exist.

Synchronous deployment may be necessary for efficient implementation ofthis method.

Blanking subframes may interfere with CQI and PMI measurements forlegacy Rel-8 UEs.

A HeNB may not use all time-frequency resources available to it for itstransmissions. If a macro cell knows the subset of the time-frequencyresources used by the HeNB (or an approximation of it), interference canbe minimized by macro cell avoiding the use of those resources fortransmission to/from UEs near the HeNB. The main principle is to makethe subset of the resources used by a HeNB a function of its cellidentifier. In this LTE context, a cell identifier can be a physicalcell identifier (PCID) or global cell identifier (GCID). Thus when a UEsends a measurement report indicating the presence of a HeNB withidentifier X, the macro cell is able to determine which time-frequencyresources it should preferentially use to minimize interference whentransmitting to the UE.

The time-frequency resources required by a HeNB vary with the cellloading (more resources are used when more UEs are connected to thecell). Therefore, to allow more accuracy in interference minimization,the following concepts are proposed:

A HeNB uses resources in a well defined order; that is, sets ofresources R1, R2, R3 . . . , have a logical ordering such that R2 isused for scheduling only if R1 is already in use, R3 is used only if R2is already in use, and so on.

A “load indicator” is included in the measurement report sent by the UE.The load indicator may be a measurement performed by the UE indicatingthe cell loading status, or information transmitted by the HeNBindicating its loading status. The load indicator along with theordering defined above, enable the macro cell to get a better estimateof the resources that it should preferentially avoid.

Three embodiments of the present invention are described below.

In the first embodiment of the invention, a HeNB has a mapping functionf( )→resources that it is allowed to use for all of its dynamic datascheduling. The mapping function maps at least one of an identity suchas the PCID, a parameter related to the cell load, etc. to theresources. For example, if f(PCID, load)→resources can define themapping function, where the load can just be equal to the number of UEsconnected to the HeNB. Effectively, a scheduler pattern intime-frequency is defined as a function of PCID and the HeNB is allowedto use a subset of the resources in the scheduler pattern determined byscaling rules as a function of the load. The serving macro cell requeststhe UE to measure and report RSRP levels and the relative timing ofneighboring cells (consisting of one or more HeNBs). The macro cellrequests the UE to measure RSRQ on HeNBs using a pre-defined or asignaled pattern with time reuse over which measurements are carried out(e.g., pattern defined over 10 ms periodicity). This pattern wouldcomprise a set of subframe number+OFDM symbol number+subcarriercombinations over which the UE is expected to carry out the RSSImeasurements intended for its RSRQ or RS-SINR report. The idea is that,by configuring RSRQ or RS-SINR measurements over a certain subset ofsymbols in a radioframe for each HeNB, the serving cell can set up asystem of equations to solve for the average loading in each of theHeNBs over the measurement duration. The serving macro cell uses theRSRQ or RS-SINR report from the UE to detect what the loading is in theHeNB (and therefore determine the subset of the scheduler patternavailable for DL scheduling in the HeNB).

Once the serving cell is able to detect what the DL scheduler pattern isfor the HeNB that the UE it is serving is close to, the serving cell canconfigure its DL scheduling resources orthogonal (or almost orthogonal)to this pattern to reduce DL interference from HeNBs.

The mapping function can be generalized to utilize one or more of PCID,GCID, TAC, CSG identity, etc.—f(PCID,GCID, TAC, CSG identity,load)→resources.

The mapping function can also utilize a frequency index (such as asubcarrier offset or resource block index) or a time index (such as aslot number, a subframe number or a frame number).

A separate mapping function that utilizes one or more of PCID, GCID,TAC, HeNB identity, CSG identity, etc. together with the DL/UL BW of theCSG cell and the duplexer separation can be used to determine an ULscheduler pattern that the CSG cell may use—g(PCID,GCID, TAC, CSGidentity, load, DL BW, UL BW, duplexer separation, frequency_index,time_index)→resources.

The HeNB preferentially or mandatorily uses the resources indicated bythe mapping for scheduling. The macro eNB excludes from its DLscheduling to the UE its DL resources that overlap with the resources ofthe HeNB's DL indicated by the mapping. If the HeNB transmissionbehavior is preferential (i.e., not mandated), there would be only begains in the statistical sense. On the other hand, if the HeNB celltransmission behavior is mandated, the transmissions are orthogonalized,and the DL interference to UEs both in macro and HeNBs is avoided.

In the second embodiment of the present invention, the networkimplementation allows transmission of a “load indicator” in the systembroadcast of a HeNB. The serving macro cell and the HeNB can map thecell identity (e.g., PCID) and load indicator to a subset of resources:f(PCID, load-indicator)→resources. The UE can send a measurement reportlisting the HeNB PCID/GCID, the HeNB's frame timing say, as an offsetrelative to the frame timing macro cell and the load indicator. In onefurther embodiment, a UE can to periodically read system broadcast(e.g., Master Information Block, System Information Block, etc.) ofnon-serving HeNBs. As a further alternative, the UE can read thetime-frequency reuse pattern (or HeNB data transmission “zone” intime/frequency) used by the HeNB in its system broadcast. Mechanisms toreduce the system information broadcast reading of the non-serving HeNBare possible. For example:

-   -   A UE reads system information broadcast only if HeNB RSRP is        greater than macro cell RSRP by an offset; where the offset can        be a function of macro RSRP or can be configured by the serving        macro cell.    -   A UE reads system information broadcast only if some HeNB RSRP        criterion is met for a certain duration of time.    -   A UE reads system information broadcast only if path loss from a        HeNB is less than a threshold.

In a third embodiment of the present invention corresponds to aheterogeneous deployment with transmitters with different operatingpowers. The method of UE association to base stations based on a signalto interference ratio (SIR) metric such as reference signal-signal tointerference and noise ratio (RS-SINR) or reference signal receivequality (RSRQ) has advantages over RSRP based association. For a UE inconnected mode, the serving eNB can configure the UE for measurementreporting (event triggered or event triggered periodic or a periodicmeasurement) of a SIR metric. For a UE in idle mode, the serving eNB canconfigure the UE to trigger a reselection evaluation when the SIR metricfalls below a certain configured threshold. FIG. 4 presents aspects ofmethods of this third embodiment.

One can envisage the following options for a SIR metric.

RSRQ is used as the SIR metric, where RSRQ is defined as follows.

${{RSRQ} = \frac{P_{CRS}}{P_{CRS} + L_{serv} + \left( {I + N} \right)}},$

where P_(CRS) is the received pilot power, is the received power ofserving cell transmission outside of CRS (also referred to as “load”here) and (I+N) is the total interference plus noise.

The UE behavior consists of estimating RSRQ and if RSRQ is below athreshold (signaled) then this event is triggered and UE sends ameasurement report.

RS-SINR is used as the SIR metric where RS-SINR is defined as

${{SINR} = \frac{P_{CRS}}{\left( {I + N} \right)}},$

may be a better measure as it overcomes some issues raised earlier.Further, a Layer 3 filtering applied to RS-SINR measurements, may behelpful in ensuring that the interference is persistent. In general, twoapproaches are used for estimating (I+N).

In the first approach, the channel is estimated for pilot REs and anestimate of the variance of the residual signal on those REs gives us an(I+N) estimate.

In the second approach, if a codeword is correctly decoded, then anestimate of the variance of residual signal gives us an (I+N) estimate.

The UE behavior consists of estimating the RS-SINR and if RS-SINR isbelow a threshold then this event is triggered and UE sends ameasurement report.

Channel Quality Information (CQI) is used as the SIR metric where CQI isdefined as the highest MCS level that can be transported on a trafficchannel such that the block error rate (or packet error rate) is below acertain threshold (for example, a error rate threshold of 10%). For CQIcomputation, the reference signal that is used for demodulation(cell-specific reference signal or dedicated reference signal, dependingon the transmission mode configured) is used for estimating the channelstate and interference. If the transmission is carried out on a certaintime-frequency repetition pattern (indicated by the measurementpattern), the estimated channel state and interference measurement overthe time-frequency pattern is used for CQI computation. The serving eNBcan configure a UE to measure CQI as described above and report it backas a periodic or as an event-triggered measurement. When the eNB is onlytransmitting on time-frequency resources as implied by the pattern, theCQI report provides means for supporting link adaptation.

Hypothetical PDCCH block error rate is used as the SIR metric. Referencesignals are used for estimating the channel state and interference. Acertain downlink control information (DCI) format is hypothesized as thetransmitted control channel and the corresponding block error rate isestimated. The block error rate of a hypothetical PDCCH format caneither be reported back to the serving eNB as a periodic report or on anevent trigger to help the eNB determine if there is a PDCCH bottleneck.Alternately, the PDCCH block error rate can be used for generatingout-of-sync and in-sync indications as part of the radio link monitoringprocess. In one embodiment, if the PDCCH block error rate exceeds acertain threshold for a certain duration of time, a radio link failurecan be triggered by the UE. In idle mode, if the PDCCH block errorexceeds a pre-determined threshold, the UE can trigger a reselectionevaluation. The UE can also rank the cells available for reselectionbased on the PDCCH block error rate metric.

There are two problems associated with the RSRQ metric as it is definedin LTE Rel-8. Although, RSRQ is defined as a measure of DL signalquality, one of the problems associated with this metric arises from theway it has been in defined in Rel-8 (TS 36.214). The RSSI measurement,as part of the RSRQ computation, is required to be performed on the sameset of resource blocks as that used for measuring RSRP. Since, the usedmeasurement bandwidth can be anywhere from 6 PRBs toallowedMeasBandwidth (defined in TS 36.331) depending on thevendor-specific implementation, the measured RSRQ can either be anarrow-band measurement or a wideband measurement or something inbetween. Further RSSI needs to be measured on CRS-bearing OFDM symbolsonly, and TS 36.214 does not specify which subset of those symbols needto be used (i.e., should the measurement be carried out either on thecontrol region only or the data region only or on both). In TS 36.214,RSRQ is however defined only for connected mode. RSRQ in idle mode canre-defined as wideband measurement to more accurately reflect the DLsignal conditions. However, even with this, there are potential problemsassociated with this metric in the following two scenarios.

In a first scenario, large macro-cell load variations will result in anRSRQ threshold configured conservatively (i.e., to a low value)—this maylead to large fraction of paging outage undetected.

In a second scenario, when HeNBs are deployed on a partial BW (e.g., 5MHz HeNB in a 10/20 MHz band, as shown in FIG. 3), RSRQ (or evenwideband RS-SINR) is not a good measure of paging reliability.

The first Scenario is further elaborated in the sequel. It was notedearlier that one of scenarios where RSRQ is problematic is where theserving cell load variations are large. For example, a UE is connectedto a macro-cell that has a time-varying load will result in largevariations in the estimated RSRQ. The measured RSRQ can be low wheneither the serving cell load is large or when the interference is largeor both. It is difficult to distinguish between the following two caseswith a single RSRQ threshold: high serving cell load, but lowinterference from HeNBs; and low serving cell load, but highinterference from HeNBs.

A reselection should be triggered only when the HeNB interference islarge (case b)) and not when the serving cell load is high but theinterference is low (case a)) as a typical eNB scheduler wouldprioritize paging and SI-x transmission over user traffic even underhigh loading conditions.

In order that the configured RSRQ threshold (e.g., Sintrasearch orSnonintrasearch) does not lead to unnecessary reselections and excessivepower consumption, the network may end up setting a RSRQ threshold thatis conservative (i.e., to a low value with a large hysteresis). This maylead to large fraction of paging outage undetected.

The second Scenario is further elaborated in the sequel. As mentionedearlier, the other problem associated with RSRQ as a quality metric iswhen HeNBs are deployed on a partial BW (e.g., 5 MHz HeNB in a 10/20 MHzband, as shown in FIG. 3). With techniques such as adaptive carrierpartitioning being considered for Rel-9 HeNB deployments forinterference coordination, it is likely that the many HeNB deploymentswill be on partial BW.

When a macro-cell UE roams close to a partial BW HeNB and experienceslarge interference on only a subset of the RBs, paging channel can bereliably received by one or more of the following steps.

-   -   1) A Rel-9 UE can employ per-subband interference estimation for        both PDCCH and PDSCH decoding to alleviate the effect of a        narrowband interferer    -   2) The serving eNB can transmit paging PDSCH outside of the        jammed RBs (i.e., outside of the HeNB-occupied BW) if all HeNBs        in its footprint are allowed to use only a part of the BW        available to the serving macro-eNB.    -   3) On the other hand, if the serving eNB does not know a priori        which portion of its BW is getting jammed from the HeNB (for        example, when adaptive frequency partitioning is used) as the UE        is in idle mode and its location is unknown to the serving eNB.        In this scenario, the serving eNB can schedule paging PDSCH on        different portions of the BW on different paging occasions so        that there is at least one paging occasion where the UE can        receive the paging PDSCH on a subband not jammed by the        interfering HeNB. Alternately, paging PDSCH scheduled on DVRB        allocations with sufficiently low code rate can provide the        necessary frequency diversity when the UE uses per-subband        interference estimation.

The combination 1)+2) or 1)+3) is likely sufficient to address theproblem of a partial BW interferer. Clearly, the RSRQ metric, even whendefined as a wideband measure is not a good measure of pagingreliability if the methods described above are made use of in Rel-9. Themain issue is that the measured RSRQ can be low even when the pagingchannel is quite reliable. Therefore, a reselection may be triggeredunnecessarily even when the UE can be paged reliably thereby leading toincreased loading and excessive battery drain.

Towards addressing the issues raised in scenario 1 and scenario 2, twosolutions are proposed to enhance the RSRQ metric. According to a firstsolution, the serving eNB transmits a measurement pattern whichcomprises a set of resource blocks possibly spread over multipletimeslots possibly repeating in time or frequency or both (i.e.,time-frequency reuse). The UE is required make RSSI measurements overthe RBs indicated by the measurement pattern. For example, when adaptivefrequency reuse is implemented where the overlay macro-cell is segmentedand the HeNBs use one of the segments, a UE that roams close to a HeNBcan experience high interference on the segment that is occupied by theHeNB while the signal quality can be good on the rest of the bandwidth.The serving eNB can configure the UE for RSRQ measurement where the RSSIneeds to be measured on the segment not occupied by the HeNB. For thispurpose, the serving eNB can signal the set of resource blocks or theset of subcarriers that the UE should exclude from its measurements.

According to a second solution, the serving eNB maintains a constantload over the measurement pattern so that the loading from the servingeNB is factored out in the RSRQ measurement. In other words, by suitablychoosing an RSRQ threshold that is aware of the serving eNB load on themeasurement pattern, the RSRQ metric can be made equivalent to theRS-SINR metric.

These enhancements can effectively address the problems raised inScenario 1 and Scenario 2.

For idle mode operation, the following aspects are noted. Triggering ofreselection evaluation based on a RS-SINR or RSRQ criterion can beuseful. Ranking of candidates based on a RS-SINR or RSRQ criterion andreselection to the “best” allowed cell based on this criterion can beuseful. The loading in different potential candidate cells and theinterference might be time-varying. The cell on which the UE is campedcan signal a measurement pattern (e.g., time-frequency resources withcertain re-use factor) over which the UE is asked to measure the RSSI orthe interference for a given cell (serving or neighbor cell) in itsRS-SINR or RSRQ computation. In particular, it can be specified that theUE measure load/interference on just the data region corresponding toeach cell. Each neighbor cell might have data regions corresponding todifferent time-frequency resources relative to the serving cell if thedeployment is asynchronous. If RSRQ is used are measurement, the servingeNB can maintain a constant load on certain time frequency resources.For example, a constant load is maintained over a measurement patterncomprising two consecutive subframes (2 ms) separated by 50 ms. The loadcan be constant over all the RBs in these subframes or over just asubset of the RBs available (so called, “constant load region”). Thispattern can be signaled to UEs over SIB or RRC. The UE measures RSSI,that includes both the interference from co-channel neighbor eNBs, noiseand the transmission from the serving eNB (load) over the constant loadregion indicated by the measurement pattern. This way, by a suitablethreshold that is serving-cell load-aware can be configured. In thiscase, the RSRQ metric is equivalent to a SINR metric that is independentof the serving cell load.

In one embodiment, the UE may autonomously detect the bandwidth of aninterfering HeNB that is in range by reading the master informationblock (MIB) or system information broadcast (SIB) transmission from theHeNB. This allows the UE to identify the set of subcarriers that itneeds to exclude from interference measurement in the computation of theSINR metric. Specifically, for RSRQ, the UE can exclude the bandwidth orthe set of subcarriers occupied by the HeNB in RSSI measurements so thatthe RSRQ reflects the quality of the channel within the bandwidthoccupied by the serving eNB that excludes the bandwidth occupied by theHeNB. In particular, when the UE (referred to as macro-UE or MUE in thesequel) is in idle mode, this might mean missed paging and theassociated consequences (e.g., missed network-originated calls, etc.).The problem becomes especially severe when the MeNB signal is weak inscenarios where the HeNB is far away from the MeNB (e.g., HeNB close tothe macro-cell edge). Techniques for adaptation of DL transmit power bythe HeNBs along the same lines as that adopted in TR 25.967, “Home NodeB Radio Frequency (RF) Requirements (FDD) (Release 9)” are likely to beinvestigated further for mitigating this problem. However, this alonemight not be sufficient. Several approaches have been considered in theLTE context in both RAN WG2 and WG4 to specifically address thisproblem. One approach is based on HeNB signaling of an intra-frequencyreselection indicator (IFRI) bit in its system broadcast which wouldtrigger an inter-frequency reselection and barring of the serving cellfrequency layer when the MUE approaches the HeNB R2-092416. Anotherapproach is based on the incorporation of RSRQ-based reselectiontriggering mechanism together with the existing RSRP-based mechanism inRel-8. In the sequel, we discuss these methods further.

In R2-092416, a mechanism where a UE camped on a MeNB attempts toreselect to a different carrier, i.e., performs inter-frequencyreselection upon detecting that the intra-frequency reselectionindicator (IFRI) bit is set by a HeNB that is on the same carrier as theMeNB. This method can be conditionally enabled when the UE roams closeto the signaling range of the HeNB based on the pathloss measurements.The IFRI bit, if set, and if the HeNB is the strongest cell, the UEattempts an inter-frequency reselection and/or bars the shared carrierfrequency for a fixed duration (e.g., 300 sec) from reselection.However, the need to read IFRI bit means that every UE is required todecode the SIB transmission from the HeNB (when it detects that it isclose to a HeNB). This is an added processing requirement on the UE inidle mode which adds to power consumption particularly when the UE candeduce that the HeNB belongs to closed subscriber group from itsphysical cell identity (PCID). Apart from this issue, it has been notedin R4-091896 that, even with DL power adaptation, there are still alarge percentage of users that either unnecessarily trigger areselection when the signal conditions are still good or do not triggera reselection even when the signal conditions severely deteriorate whenthe IFRI method used.

In R4-091895, it has been suggested that adding a RSRQ-based triggeringmechanism in addition to the existing RSRP-based triggering in Rel-8 cansignificantly mitigate the problem. In this proposal, RSRQ is used as ametric for detecting a DL signal quality problem arising out of largeinterference from HeNB transmissions. The UE is required to monitor bothRSRP of the serving cell and the RSRQ, and if either metric drops belowthe respective thresholds, a reselection evaluation is triggered.

Although, RSRQ can be used as indication of the DL signal quality, oneof the problems associated with this metric arises from the way it hasbeen in defined in Rel-8 TS 36.314 v8.6.0, “PhysicalLayer—Measurements”. The RSSI measurement, as part of the RSRQcomputation, is required to be performed on the same set of resourceblocks as that used for measuring RSRP. Since, the used measurementbandwidth can be anywhere from 6 PRBs to allowedMeasBandwidth (definedin TS 36.331) depending on the vendor-specific implementation, themeasured RSRQ can either be a narrow-band measurement or a widebandmeasurement or something in between. In TS 36.314 v8.6.0, “PhysicalLayer—Measurements”, RSRQ is however defined only for connected mode.RSRQ in idle mode can re-defined as wideband measurement to moreaccurately reflect the DL signal conditions. However, in the nextsection, we discuss why even with this, there are potential problemsassociated with this metric. In summary, a poorly defined RSRQ leads tothe following problems. First, large macro-cell load variations willresult in an RSRQ threshold configured conservatively (i.e., to a lowvalue)—this will lead to large fraction of paging outage undetected.Second, when HeNBs are deployed on a partial BW (e.g., 5 MHz HeNB in a10 or 20 MHz band), RSRQ (or even wideband RS-SINR) is not a goodmeasure of paging reliability. Some, wideband CQI mechanism withfrequency selective interference estimation seems necessary.

However, the problem described with RSRQ can be circumvented as follows.To address the concerns we raised earlier with RSRQ, the followingadditional methods can be adopted.

The macro-eNB can ask the UE to only measure load+interference for RSRQmeasurement on a certain pre-determined set of RBs and certainsymbols—currently whether the UE uses data region or control formeasuring RSRQ or whether it uses narrow band or wide band isunspecified. The macro-eNB can maintain a constant load on a subset ofRBs (i.e., on some RBs on some subframes) so that it can configure anRSRQ threshold based on the expected serving cell load. This removesload dependence of RSRQ.

The network operations and management (O & M) might know which subbandHeNB are allowed to deploy themselves on (e.g., one preferred HeNBdeployment approach is to use 5 MHz for HeNBs out of 20 MHz in amacro-cellular overlay). So, the RBs where interference from non-allowedCSG or hybrid cells are present can be configured for measurement.Alternately, it may be better to signal the RBs that the UE shouldexclude from load measurements rather than signal the set of RBs tomeasure on as the macro-eNB would likely allocate paging PDSCH outsidethat region. Signaling the set of RBs to be excluded may be preferablebecause measurement performed on the center 6 PRBs on symbols that bearsynchronization (SCH) or Physical Broadcast Channel (PBCH) in asynchronous deployment are not indicative of the HeNB interference or ofpaging channel quality.

In connected mode, the UE can choose certain RBs and certain symbolswhere it measures load and interference in order to calculate RSRQ. TheUE can then send a measurement report that includes the calculated RSRQand the RBs and symbols over which the load and interference aremeasured. The eNB, based on its knowledge of loading in the reported RBsand symbols can estimate the additional interference caused by the HeNB.

Finally, if a fixed timing offset is used for all HeNBs relative to themacro-eNB (our control channel protection proposal) that the network(NW) is aware of, this offset can be signaled to the UE so that it canmeasure RSRQ only on the data region.

Paging channel is addressed by physical downlink control channel (PDCCH)format 1C. PDCCH is the likely bottleneck in both coverage-limited andinterference-limited scenarios. Therefore, if the UE were to reliablypredict the performance of paging, it can then use this as the basis foridentifying DL signal quality problems where the paging channel islikely to fail. There are a few reasons why accurate paging performanceprediction is feasible.

Link quality prediction methods like EESM, MMIB, etc. have been shown tobe quite accurate in estimating the block error rate (BLER) of codedpacket transmissions in an OFDM link. These methods can naturally foldin the impact of transmit antenna configuration (SIMO, SFBC orSFBC-FSTD), PDDCH power boost, number of control symbols and CCEaggregation level used into the performance of PDCCH DCI format 1Ctransmission.

In heterogeneous deployments where there are HeNB transmissions withpartial BW overlap, frequency selective interference on the DL can beestimated on subband basis. The impact of narrowband interferers onwideband PDCCH signaling can be accurately captured if subbandinterference estimation were to be used.

PDCCH DCI format 1C performance is indicative of paging channelperformance in most scenarios. DCI format 1C transmission qualityprediction can be carried in a manner similar to what is currently beingdone for in-sync detection as part of radio link monitoring in Rel-8.The block error rate (BLER) associated with a hypothetical 1Ctransmission can be computed from RS-SINR evaluated on a subcarrier or asubband level. The transmission parameters associated with thehypothetical PDCCH can be chosen to be either the best case parameters(e.g., +4 dB PDCCH power boost) or normal case transmission parameters(e.g., 0 dB PDCCH power boost) similar to what was adopted for in-syncdetection in radio link monitoring.

When a UE connected to a macro-eNB is within the interference range of aHeNB that is deployed on a partial bandwidth relative to the macro-eNB(e.g., 5 MHz HeNB deployed on a overlay macro-eNB network with 10 MHzbandwidth), a part of the macro-eNB bandwidth is always blocked by highinterference from HeNB if the UE is close to the HeNB. In such ascenario, a frequency-domain pattern indicating the bandwidth occupiedby the HeNB can be made use of in interference estimation as part ofpaging channel BLER prediction.

With the enhancements to RSRQ measurement proposed in this embodiment,the RSRQ metric measured at the UE can be used as a reliable means topredict the paging channel performance. The enhanced RSRQ method may beused as sub-optimal alternative to paging channel prediction based onhypothetical PDCCH block error rate estimation. The following furtherembodiments can be envisaged with the enhanced RSRQ approach.

In a first embodiment, both intra-frequency reselection evaluation andinter-frequency reselection evaluation can be triggered if therespective S-conditions (determined by the thresholds S_(intrasearch)and Snonintrasearch respectively) can be triggered with the enhancedRSRQ. Both coverage limited scenarios (due to low RSRP at cell edge) andinterference-limited scenarios (high interference due to a nearbyopen-access or a closed CSG) can be detected by enhanced RSRQ, however,it might be desirable for network operators to dimension their cell sizebased on RSRP. So, enhanced RSRQ may be used in addition to RSRP inreselection. For example, a reselection may be triggered if either RSRQfalls below a first threshold or if RSRP falls below a second threshold.

In a second embodiment, when other radio access technologies (RATs) suchas GSM/EGPRS, WCDMA, CDMA 2000 1x/HRPD, etc. are configured as inter-RATlayers or when LTE is configured on other carrier frequencies as aninter-frequency carrier with priorities different from the serving cellpriority, in Rel-8, Thresh_(x,high), Thresh_(x,low),Thresh_(serving,low) are used as thresholds triggering reselection to ahigher priority layer or a lower priority layer. The measurement used isRSRP. This can be easily extended to enhanced RSRQ, where a reselectionor a reselection evaluation is triggered if the enhanced RSRQmeasurement falls below the relevant threshold. When non-intra frequencyhot spot with partial BW overlap interferes with macro UEs, wideband CQIwith frequency selective interference measurement can detect if there ispaging outage or not.

In a third embodiment, when the UE is in a coverage-limited region, anout-of-service area state is activated based on the measured RSRPfalling below a certain pre-determined threshold. The same is applicableto RSRQ. If RSRQ drops below a threshold for a certain duration of time,the UE can declare an out-of-coverage area event and display this stateon its screen visible to the user.

In a fourth embodiment, when the UE is in the presence of a non-allowedCSG cell, the event can be detected by comparing the measured RSRQagainst a threshold. In this event, three options are possible. How tohandle non-allowed CSG cell. A first option is barring when it is thebest cell (after evaluation of S_(intrasearch)). A second option is noparticular behaviour (out-of-service area or Thresh_(serving,low) usingRSRQ could detect problem). A third option is deprioritization if it isthe best cell with some offset.

In a fifth embodiment, once a reselection evaluation is triggered, inRel-8, the common approach is to measure RSRP for LTE cells on layerswith equal priority and rank them. A reselection is performed to thebest cell if it happens to remain the highest ranked for a certainduration of time (Treselection). However, enhanced RSRQ can also be usedfor ranking instead of RSRP.

In the previous embodiments, the serving eNB transmits a measurementpattern indicating a set of resource blocks (RBs) over a certain numberof timeslots that can possibly have a certain time-frequency reuse. Onetimeslot can correspond to one OFDM symbol (i.e., 71 us in time fornormal cyclic prefix), or one slot (0.5 ms) or one subframe (1 ms) orsome other time interval. The set of RBs can be one RB (12 subcarriers)or can span the entire downlink bandwidth. The pattern can be formed byrepeating a basic pattern in frequency (frequency reuse) or in time(time reuse) or both in time and frequency (time-frequency reuse whichis the more general case). Therefore, the pattern comprises a set of RBsspread across a number of timeslots. In one embodiment, the measurementpattern can be a set of contiguous physical resource blocks with afrequency offset and a second parameter corresponding to the number ofRBs in the pattern. This pattern can repeat in time once every subframeresulting in a time reuse factor of 1 ms in E-UTRA. In anotherembodiment, the measurement pattern can be a set of non-contiguousresource blocks (with a bit-map association to the RBs). In a thirdembodiment, the measurement pattern can be derived from a zero-onevalued matrix of size M×N, where M is the number of RBs available infrequency domain and N is the number of RBs available in time domain(e.g., number of slots or subframes). The time reuse factor for thispattern corresponds to the amount of time required for transmission of NRBs in time domain. As one example of this embodiment, the zero-onematrix is a submatrix of a P×P permutation matrix, where P=max{M, N}.The term time-frequency reuse implies a certain repetition interval forthe basic pattern on a time-frequency grid. Frequency-only reuse is aspecial case of this wherein a basic pattern is repeated in frequencydomain only. Time-only reuse is another special case wherein a basicpattern is repeated in the time domain only.

While the present disclosure and the best modes thereof have beendescribed in a manner establishing possession and enabling those ofordinary skill to make and use the same, it will be understood andappreciated that there are equivalents to the exemplary embodimentsdisclosed herein and that modifications and variations may be madethereto without departing from the scope and spirit of the inventions,which are to be limited not by the exemplary embodiments but by theappended claims.

What is claimed is:
 1. A method in a wireless terminal transceiver, themethod comprising: receiving, at the wireless terminal transceiver, asequence of frames from a first base station, each frame in the sequencecontaining a first set of time-frequency resources which are used forscheduling data and a second set of time-frequency resources not usedfor scheduling data; receiving, at the wireless terminal transceiver, amessage from the first base station identifying a third set oftime-frequency resources that is a subset of the first set oftime-frequency resources; estimating a channel state based on a sequenceof frames received by the wireless terminal transceiver from a secondbase station that coincide with the third set of time-frequencyresources; wherein estimating a channel state comprises estimating areference signal received power of the first base station.
 2. The methodof claim 1 wherein estimating a channel state comprises estimating areference signal received power of the second base station.
 3. Themethod of claim 1 further comprising sending a message to a serving basestation including at least one of a reference signal received power or asignal to interference ratio metric.
 4. The method according to claim 1,wherein the time-frequency resources are symbol durations, time-slots,subframes or frames.